Test performed at 80°C and 60°C, With Closed Cathode, Imposed Gauge Pressure 0.5bar,

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6.2.4 Test performed at 80°C and 60°C, With Closed Cathode, Imposed Gauge Pressure

122 Modelling at 80°C

Nyquist plot below shows the impedance measurement at 80°C, both potentiostatic and galvanostatic. Impedance spectra under voltage control are characterized by an arc at high frequency and a straight line/second incomplete arc at mid-low frequency (some tests were interrupted before reaching the lowest frequency due to noise, this explains the difference in shape at lower frequencies). On the other hand, under current control the shape changes as the DC value is increased. At 0 mA and 1000 mA there are three arcs, one at HF and the others at mid-low frequency. As the DC current is increased up to 2000 mA the arcs become two and the LF arc tends to reduce.

Figure 99 Nyquist plot under current control at 80°C 0.5bar closed cathode with 20% pump.

Figure 98 Trend of pressure during potentiostatic and galvanostatic measurements.

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Figure 100 Zoom of Nyquist plot under current control at 80°C 0.5bar closed cathode with 20% pump.

Figure 101 Nyquist plot under voltage control at 80°C 0.5bar closed cathode with 20% pump.

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Figure 102 Zoom of Nyquist plot under voltage control at 80°C 0.5bar closed cathode with 20% pump.

Modelling at 60°C

Figures below show the EIS measurements at the end of the test at 60°C.

Figure 103 Nyquist plot under voltage control at 60°C 0.5bar closed cathode with 20% pump.

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Figure 104 Zoom of Nyquist plot under voltage control at 60°C 0.5bar closed cathode with 20% pump.

Figure 105 Nyquist plot under current control at 60°C 0.5bar closed cathode with 20% pump.

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Figure 106 Zoom of Nyquist plot under current control at 60°C 0.5bar closed cathode with 20% pump.

Impedance spectra under voltage control are characterized by an arc at high frequency and a straight line/second incomplete arc at mid-low frequency. The low frequency features shows a dependence on voltage: as the DC value of potential increases, this tends to bend down towards the real axis. In fact, at 1.5 V a second arc is clearly visible. Same consideration for tests performed under current control, i.e. the shape changes as the DC value is increased.

DISCUSSION

Considerations on tests performed at 80°C

Results obtained by fitting the impedance spectra measured under current control are analysed from a qualitative point of view. First, it is observed a dependence on current density principally related to the LF arc. As the current density increases it is noticed that the LF arc is characterized by a decrease of charge transfer resistance while its capacitance C increases; so the LF semicircle becomes smaller. Considering that in the range 0-2 A (so 0-0,08 A/cm2 current density) we are in the activation domain and that the LF arc shows a dependence on current density, it is suggested that the LF arc is controlled by the charge-transfer kinetics. It means that in the Nyquist plot the LF arc is related to OER and HER. The presence of noise makes impossible to say if mass transport processes are present.

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Instead, the HF arc and the ohmic resistance show an almost constant trend. Hence, it is possible to assess that the HF arc is current independent. This means that is not related to kinetics processes but rather to the MEA structure.

About potentiostatic tests, the analysis of the fitting results concerns only valid spectra which are those obtained in the range 0.5-1 V and it is focused on the HF region of the spectra where quality is higher. It is possible to notice that the ohmic resistance is not varying significantly with voltage (from 0.770 to 0.730 Ωcm²), the same thing for the HF arc which remains almost constant in this range. In fact, the maximum frequency is constant at 251.2 Hz. Figures below show a comparison between ECM fit and Circle fit. The trend is similar for both types of fit. In the ECM fit mode the result at 0.8 V is considered an outlier due to higher error of the fit.

Figure 107 Trend of the HF parameters at 60°C with 20%pump.

Considerations on tests performed at 60°C

The analysis is focused on the EIS measurements performed at the end of the test. Only good impedance spectra are considered, thus 1500 mA and 2000 mA for the galvanostatic mode and the range 0.5-1.5 V for the potentiostatic mode.

As the current increases in the range 1.5-2 A we have that at LF the charge transfer resistance decreases from 0.583 to 0.428 Ωcm² and the capacitance C is almost constant at 4-4.1 F, so the LF semicircle becomes smaller. Instead, the HF arc in the same range is characterized by an almost constant value of the charge transfer resistance HF (from 0.37 to 0.39 Ωcm²) and capacitance (from 0.034 to 0.03 F), whereas the ohmic resistance is almost constant around 1.25-1.24 Ωcm².

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Considering the range 0.5-1.5 V, it is possible to notice that the ohmic resistance is not varying significantly with voltage (from 1.178 to 1.200 Ωcm²), whereas as the voltage increases, we have that the LF arc bends down to the real axis. Moreover, at LF the charge transfer resistance decreases and the capacitance C increases, so the LF semicircle becomes smaller. The high frequency arc, characterized by Rct and C, shows a very slight decrease with voltage: in particular, capacitance is increased while the charge transfer resistance shows lower value with higher voltage level.

Figure 108 Trend of the HF parameters at 80°C with 20%pump.

Comparison Between 80°C And 60°C At 20% Pump

Polarization curves show that the performance of the cell improves at higher temperatures.

This result is in accordance with the findings of the EIS tests. First of all, the trends in Figure 110 show that the HF feature is improved at higher temperature. Further on, a qualitative comparison between impedance spectra obtained with galvanostatic tests allows to say that total polarization resistance is smaller at 80°C, hence not only the electrolyte benefits from higher temperature but also the anode.

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Figure 109

Figure 110 Comparison between 80°C and 60°C of the HF parameters with 20%pump.

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6.2.5 Test Performed At 60°C, Imposed Gauge Pressure 0.5bar, with 10% Mass Low Rates